In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium, to form characters and image. The ink-jet printing methods have been rapidly spread because of their various advantages such as easiness of full coloration, low costs, capability of using plain paper as the recording medium, non-contact with printed images and characters, etc. Among such printing methods, in particular, in view of enhancing the weather resistance and water resistance of printed images and characters, an ink-jet printing method utilizing an ink containing a pigment as the colorant has now come to dominate.
For example, WO 00/39226 discloses a water-based ink containing a pigment-containing vinyl polymer which is in the form of a graft polymer obtained from a macromer to achieve a high image density.
JP 11-343439A discloses an ink containing a resin enclosing a colorant and a self-dispersible pigment on a surface of which at least one hydrophilic group is bonded directly or through any other atom group.
JP 2005-42005A discloses an ink formed by dispersing water-insoluble vinyl polymer particles containing carbon black in water in which the carbon black has a DBP oil absorption of 100 mL/100 g or higher and the water-insoluble vinyl polymer contains a salt-forming group.
JP 2003-231831A discloses a water-based ink using a water dispersion of particles of a water-insoluble polymer having an acid value of from 60 to 200 KOH mg/g and containing an acid group from 30 to 80 mol % of which is neutralized, and carbon black having a pH of from 1 to 6 which is incorporated in the polymer particles.
JP 2005-42098A discloses a water-based ink for ink-jet printing containing a water dispersion of water-insoluble vinyl polymer particles containing carbon black in which the carbon black contains volatile components in an amount of 5% or less as measured at 950° C. and is composed of two or more kinds of carbon blacks which are different in at least one of a primary particle diameter, a specific surface area and a DBP oil absorption from each other.
JP 2006-111691A discloses a water dispersion for ink-jet printing which contains carbon black having a pH of from 1 to 5 and carbon black having a pH of from 7 to 11.
However, the inks described in JP 2005-42098A and JP 2006-111691A tend to suffer from problems of poor storage stability such as change in viscosity of the inks with time provably owing to difference in surface properties between the respective carbon blacks used therein. In addition, these polymer-containing inks still have such an unsolved problem that the inks tend to be deteriorated in ejection property owing to increase in amount of the inks deposited on an ink nozzle plate.
JP 2001-207104A discloses a dispersion of an aqueous pigment which contains one kind of carbon black and an anionic group-containing polymer compound having a crosslinked moiety. However, since the polymer compound itself contains a crosslinking group, it is difficult to attain a sufficient crosslinking density of the polymer compound. Therefore, the dispersion tends to be still unsatisfactory in not only storage stability but also image density.